Index: /trunk/source/level-0/X86/X8632/x8632-pred.lisp
===================================================================
--- /trunk/source/level-0/X86/X8632/x8632-pred.lisp	(revision 11352)
+++ /trunk/source/level-0/X86/X8632/x8632-pred.lisp	(revision 11353)
@@ -132,4 +132,9 @@
   (single-value-return)
   @cons
+  ;; If either X or Y is NIL, lose.
+  (cmp-reg-to-nil x)
+  (je @lose)
+  (cmp-reg-to-nil y)
+  (je @lose)
   ;; Check to see if the CARs are EQ.  If so, we can avoid saving
   ;; context, and can just tail call ourselves on the CDRs.